Substantial attention has been given to human gene therapy. This term has been used to describe a wide variety of methods using recombinant biotechnology techniques to deliver a variety of different materials to a cell. Such methods include, for example, the delivery of a gene, antisense DNA or RNA, a gene encoding a cytotoxic agent, etc., by a vector to a mammalian cell, preferably a human cell either in vivo or ex vivo.
Most of the initial work has focused on the use of retroviral vectors to transform these cells. This focus has resulted from the ability of retroviruses to infect cells and have their genetic material integrated into the host cell with high efficiency. The development of a helper virus free packaging system for retrovirus vectors was a key innovation in the development of this vector system for human gene therapy. Retroviral helper virus free packaging systems generally employ the creation of a stable producer cell line that expresses a selected vector. The relatively small size of the retroviral genome, approximately 11 kb, allows for the production of a packaging cell line that synthesizes all the proteins required for viral assembly. Producer lines are made by introducing the retroviral vector into such a packaging cell line.
However, numerous difficulties with retroviruses have been reported. For example, most retroviral vectors are not capable of gene transfer to postmitotic (nondividing) cells and are thus not applicable to the nervous system because most of the cells in the adult nervous system, especially neurons, are quiescent or postmitotic. In addition, outbreaks of wild-type virus from recombinant virus-producing cell lines have also been reported with the vector itself causing a disease.
In light of these difficulties, other vector systems, such as those based on adenovirus, adeno-associated virus, and herpes simplex virus type 1 (HSV-1), are being evaluated for gene transfer into mitotic and non-mitotic cells, especially for use in non-mitotic cells, e.g., neural cells. Each system has limitations: Adenovirus vectors can only support limited long-term (2 months) gene expression, they appear to be gradually lost from neural cells, and moreover, they can cause both cytopathic effects and an immune response (Le Gal La Salle, et al., (1993); Davidson, et al., (1993); Yang, et al., (1995)). Adeno-associated virus vectors cause minimal cytopathic effects and can support at least some gene expression for up to 4 months, but gene transfer is inefficient and these vectors can accept only .about.4 kb of foreign DNA (Kaplitt, et al., (1994)). In contrast, vectors based on HSV-1 (Jenkins, et al., (1985); Ho, et al., (1988); Dobson, et al., (1990); Chiocca, et al., (1990); (Fink, et al., (1992); Frenkel, et al., (1982); Kwong, et al., (1985); Geller, et al., (1988); Geller, et al., (1990)) are attractive because HSV-1 can efficiently infect neural cells and can persist indefinitely in neurons (Stevens (1975); Roizman (1981)). Recombinant HSV-1 vectors (Wolfe, et al., (1992)) and HSV-1 plasmid vectors (Federoff, et al., (1992); Battleman, et al., (1993); Bergold, et al., (1993); Geller, et al., (1993); Ho, et al., (1993); Geschwind, et al., (1994); Xu, et al., (1994); Geller, et al., (1995)) have been used to alter neuronal physiology, and HSV-1 plasmid vectors can support some gene expression for up to 1 year (During, et al., (1994)). HSV-1 plasmid vectors contain only .about.1% of the 150 kb HSV-1 genome and have been packaged into HSV-1 particles using a helper virus, often a replication-deficient deletion mutant of HSV-1 (Geller, et al., (1990); Lim, et al., (1995)). However, the effectiveness of this vector system has been limited by a number of problems: (i) acute cytopathic effects and an immune response, largely due to gene expression from the helper virus (Johnson, et al., (1992a); Johnson, et al., (1992b); Nguyen, et al., (1992); Johnson, et al., (1994); Wood, et al., (1994); Ho, et al., (1995)) (and in part due to specific proteins in the HSV-1 particle), (ii) potential interactions between the helper virus and endogenous latent viruses, (iii) instability of gene expression (During, et al., (1994)), (iv) potential oncogenesis mediated by the helper virus [as opposed to wild type (wt) HSV-1 which is not known to cause tumors], and (v) reversion of the helper virus to wt HSV-1. The helper virus causes many of these problems and is not required after packaging, however, physical separation of the packaged vector from the helper virus has not yet been achieved. It would be desirable to have a helper virus-free packaging system for herpesvirus vectors. However, heretofore it was believed that development of such a helper virus-free system was unlikely (Lieb and Olivo (1993)).